This invention relates generally to the preparation of coatings, molding powders, fibers, films and matrix resins for composites. The invention relates specifically to the molecular level coating of metal oxide particles with polyimides and the product(s) obtained thereby.
The development of advanced high performance polymers for aerospace applications has been, and remains, a particularly active area of research. High performance polyimides have found extensive use in the aerospace industry as adhesives, and more recently as matrix resins for composites, molding powders and films.
Improvements in high performance systems are motivated by the search for advanced materials with improved or unique properties. Previous research into polyimide modifications have involved the simplest and most inexpensive methods involving manipulation of the chemical composition of mainly linear polyimides. An alternative method of modification is by incorporation of a second component of differing chemical structure or composition.
Modifications utilizing the morphology of multiphase systems with phases of differing chemical structure allows for a remarkable balance of diverse properties. This is especially true when at least one phase is on the molecular scale, allowing for a balance of diverse properties. The production of organic-inorganic hybrid materials may take place through several different methods. One route is by direct mixing of low melt glasses with engineering thermoplastics, Beall and Quinn (Phosphate glass-polymer emulsionsxe2x80x9d, Ceramic Transactions, Volume 33, 1993). Organic-inorganic blends have also been formed by intercalation of polymers in the melt between mica sheets; Giannelis (xe2x80x9cA New Strategy For Synthesizing Polymer-Ceramic Nanocompositesxe2x80x9d, Journal of the Minerals, Metals and Materials Society, Volume 44. Number 3, 1992). These processes have resulted in a class of materials called ceramers that possess properties of both inorganic glasses and organic polymers.
Organic-inorganic blends may also be produced by utilizing the sol-gel process. With this method, the inorganic phase is formed in-situ by hydrolysis and polycondensation of the alkylated metal aldoxides. Alkylated metal oxides are organic low molecular weight compounds soluble in organic solvents which precipitate as metal oxides upon condensation. Sol-gel ceramers in the past have involved the formation of transparent or translucent thin films where the organic and inorganic phases are co-mingled and then cured, as described by lyoku et al, (The Preparation of New Poly(phenylsilsesquioxane)-Polyimide Hydrid Films by the Sol-Gel Process and Their Propertiesxe2x80x9d, High Performance Polymers, Volume 6, 1994), where they indicate the formation of small particles of silicone dispersed in a film.
In another case where inorganic-organic composites are formed, the functionality of the poly(dimethylsiloxane) chains of the polymer results in strong interactions between the two components, where the polymer constitutes the continuous phase, while the ceramic material serves as reinforcing particles. When the polymer is present in lower concentrations, it becomes dispersed in the continuous ceramic phase. Mark et al, (xe2x80x9cInorganic-Organic Composites Including Some Examples Involving Polyamides and Polyimidesxe2x80x9d, Macromolecular Symposium, Volume 98, 1995), even cites cases where a bicontinuous system is formed.
Another method of generating organic-inorganic blended materials is by encapsulation. This technology is being used extensively in many industries and for a wide variety of materials. Microcapsules can have many different structures, but typically involve a core region surrounded by a shell. The geometry may be spherical or irregular, and contain a continuous core or small particles of core material surrounded by the shell. As a result of agglomeration, traditional methods of encapsulating metal oxide particles result in a multi-molecular/multi-nuclear core region surrounded by a coating. Macro Coated Particle (MCP) technology results in organic-inorganic particles in the ten to hundreds of micron range (FIG. 1). Molecular Level Coating (MLC) technology, as employed in the present invention (FIG. 2), utilizes microencapsulation technology in conjunction with sol-gel processing. The in-situ generation of the inorganic phase with MLC results in a polymer coated, molecular level, metal oxide particle in the angstrom size range.
Preparation of a ceramer by MLC results in the formation of the metal oxide as a discrete particle thinly coated with a polymer. MLC of a preceramic and a high performance polymer facilitates the design of systems that combine the thermal stability, high stiffness (modulus) or light reflective properties of a glass with the toughness and processability of a polymer. MLC further offers the advantage of metal oxide particles with less abrasive properties than uncoated metal oxides.
Titanium oxide, a commonly used whitener in pigments and coatings, is subject to weathering with long term exposure to sunlight. Exposure to ultraviolet light results in excitation of the electrons in the titanium compound which may return to the ground state by transferring free radicals to the surrounding materials. Absorption of these free radicals by the surrounding organic material leads to discoloration and degradation. In accordance with the present invention, degradation of titanium oxide is slowed by encapsulating the titanium oxide particles in a polymer that is nonreactive to free radical bombardment.
It an object of the present invention to provide a molecular level coated metal oxide particle that has less abrasive properties than uncoated metal oxide particles.
A further object of the present invention is to provide a metal oxide encapsulated with a polyimide that has synergistic property characteristics.
Another object of the present invention is to provide encapsulated titanium oxide particles to decrease the degradation and improve the weathering and colorfast property characteristics of the particles.
An additional object of the present invention is a process of encapsulation of metal oxide particles with a surrounding insulation of a polyimide to thereby insulate the metal oxide from free radical transfer and provide better weathering and good resistant color fast properties to the metal oxide.
A further object of the present invention is to provide an encapsulated titanium oxide for use in a protective urguent for human skin.
An additional object of the present invention is to provide coated metal oxide particles that hinder the loss of free radicals when exposed to ultraviolet light.
Another object of the present invention is a process of preparing molecular level coatings of metal oxide particles for use as ultraviolet and weather protectives in unguents for human skin, paper fillers, printing inks, fiber reinforced composites and textiles.
Another object of the present invention is a polymer encapsulated metal oxide matrix resin for manufacturing fiber reinforced composites wherein the metal oxide particles increase the modulus of the polymer in the composite.
The foregoing and additional objects are attained by employing, as the metal oxide coating, a polyimide having repeating units of: 
wherein Ar is a member selected from the group consisting of: 
wherein the catenation is meta, meta; meta, para; or para, para;
wherein R is a member selected from the group consisting of: 
and, wherein n is an integer in the range of 10 to 10,000.
Polymer encapsulated metal oxide particles were prepared by combining a polyamide acid in a polar aprotic solvent with a metal alkoxide solution. The polymer was imidized and the metal oxide formed simultaneously in refluxing organic solvent. The resultant polymer-metal oxide is an intimately mixed commingled blend, possessing properties of both the polymer and preceramic metal oxide.
Polymers suitable for practice of the present invention are disclosed in the following U.S. Patents (incorporated herein by reference), U.S. Pat. No. 4,094,482 (LARC(trademark) TPI); U.S. Pat. No. 4,603,061 (LARC(trademark) 6F); U.S. Pat. No. 4,937,317 (LARC(trademark) ITPI);, and U.S. Pat. No. 5,147,966 (LARC(trademark) IA), and are commercially available from NASA licensees of these patents.